JPH10227993A - Bessel beam generating method and optical scanning device using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To generate a Bessel beam with large depth of a focus by making the light beam, projected from a light source means emitting a light beam with a cylindrical light intensity distribution, incident on an image formation optical system. SOLUTION: The light source means 12 which emits the light beam (laser beam) with the cylindrical light intensity distribution consists of, for example, a vertical oscillation type surface light emission laser (semiconductor laser) and generates the laser beam (light output) 11 having the cylindrical (doughnut- shaped) light intensity distribution. Then a convex lens 22 as the image formation optical system is arranged at a distance of the focal length (-f) of the convex lens 22 from the laser end surface (light emission surface) of the vertical oscillation type surface light emission laser 12 and generates the Bessel, beam in an interference area (Bessel beam area) almost at a distance of the focal length (+f) of the convex lens 22. Thus, the Bessel beam having the intensity distribution which is nearly in proportion to the square of a 0-order Bessel function of 1st class is generated in the interference area (Bessel beam area) 21.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for generating a Bessel beam and an optical scanning device using the same, and more particularly to a method for generating a Bessel beam having a small spot and a large depth of focus and an optical scanning device using the same. It is.

[0002]

2. Description of the Related Art Conventionally, a laser beam is generally treated as a Gaussian beam and has been restricted based on its propagation characteristics. However, in recent years, a Bessel beam (Jo beam or non-diffractive beam) has attracted attention as a laser beam having a very large depth of focus and a relatively small spot diameter. For details of the vessel beam
Durnin: J. Opt. Soc. Am. A, vol. 4, No. 4, p. 651 (1987). The Bessel beam is characterized in that the light amplitude distribution in a section perpendicular to the propagation direction is proportional to the first-order zero-order Bessel function. That is, when the distance from the optical axis is r, the amplitude distribution U of the Bessel beam
(R) is represented by U (r) = A · Jo (αr) (1) Here, A and α are constants. Note that the intensity distribution can be obtained by squaring the absolute value of the amplitude distribution U (r).

As a method for obtaining a Bessel beam, for example, a method using a thin ring opening and a lens by Durnin et al. (Phys. Rev. Letters, vol. 58, No. 15, p. 1499 (198)
7)), and a method using a conical prism by Kawata and Arimoto (Preprints of the Spring Meeting of the Japan Society of Applied Physics, p.829, 30p-A-
4 (1991).

[0004]

FIG. 5 is a schematic diagram of a main part of an optical system showing a method of forming a Bessel beam using a thin ring aperture and a convex lens.

In FIG. 1, a laser beam 51 collimated by a collimator lens (not shown) is diffracted by a thin ring opening 52, passes through a convex lens 53, and forms a Bessel beam in an interference area (Bessel beam area) 54. ing. However, the Bessel beam forming method shown in the figure has a problem that the amount of light is remarkably reduced due to vignetting of the laser beam 51 when the laser beam 51 passes through the narrow ring opening 52.

On the other hand, the method of forming a Bessel beam using a conical prism has a problem that it is difficult to manufacture a conical prism. In particular, a conical prism having an apex angle close to 180 degrees has a problem that it is difficult to obtain a prism having a high angle accuracy, has a low centering accuracy, and has difficulty in obtaining the symmetry of a laser beam.

Therefore, conventionally, various means for solving the above-mentioned problems have been disclosed in, for example, JP-A-4-171415 and JP-A-4-145412.

FIG. 6 is a schematic view of a main part of an optical system showing a Bessel beam generating method disclosed in Japanese Patent Application Laid-Open No. Hei 4-171415, and FIG. 7 is an enlarged explanatory view of the light source means shown in FIG.

In FIG. 6, a laser beam 62 emitted from a semiconductor laser 61 is transmitted through a concave lens 63, a convex lens 64, and an axicon lens (axicon optical element) 65 to reduce the vignetting caused by the optical system. ) 66 forms a Bessel beam. Although the axicon lens 65 in the figure has a blazed cross-sectional shape, it has high diffraction efficiency and no loss of light amount, but there is a problem that the blazed cross-sectional shape is difficult to manufacture. When the semiconductor laser 61 is used as a light source in order to reduce the size of the entire apparatus, as shown in FIG. 7, the light intensity distribution of the laser beam emitted from the semiconductor laser 61 (near the light emission intensity distribution at the laser end face). Since the (field image) has an elliptical light intensity distribution, the light intensity distribution of the formed Bessel beam also becomes asymmetric in the x-axis direction (optical axis direction) and the y-axis direction (direction orthogonal to the optical axis). There is.

FIG. 8 is a schematic view of a main part of an optical system showing a Bessel beam generating method disclosed in Japanese Patent Application Laid-Open No. 4-45412.

In FIG. 1, an annular mirror (first optical element) 81 having a diameter d and an annular width δ, a convex lens 82 having a focal length f,
By arranging output mirrors (second optical elements) 83 in a hybrid, a Bessel beam (light output) 84 is generated. However, in the figure, there are problems that the configuration is complicated and it is difficult to reduce the size of the entire apparatus.

In order to solve the above-mentioned problems, the present invention provides a first-order zero-order light by making a light beam emitted from light source means for emitting a light beam having a cylindrical light intensity distribution incident on an imaging optical system. It is an object of the present invention to provide a Bessel beam generating method capable of easily generating a Bessel beam having an intensity distribution substantially proportional to the square of the Bessel function with a simple optical system, and an optical scanning device using the same.

[0013]

According to the present invention, there is provided a method for generating a Bessel beam, comprising the steps of: (1) causing a light beam emitted from a light source means for emitting a light beam having a cylindrical light intensity distribution to enter an imaging optical system; It is characterized by generating a Bessel beam having an intensity distribution substantially proportional to the square of the first-order zero-order Bessel function.

In particular, (1-1) the light source means comprises a vertical oscillation type surface emitting laser, a light emitting diode (LED), or a super luminescent diode (SLD); That the optical system has a convex lens, (1-3) that a beam expander or a relay lens is arranged in the imaging optical system, (1-4)
The convex lens is disposed at a position away from the light emitting surface of the light source means by a focal length of the convex lens, and the Bessel beam is generated near a position away from the focal length of the convex lens.

(2) A light beam emitted from a light source means for emitting a light beam having a cylindrical light intensity distribution is converted into a first kind of zero-order Bessel function of the first kind via an imaging optical system. A Bessel beam having an intensity distribution substantially proportional to the power, the Bessel beam is condensed by a condensing unit and made incident on a deflecting unit, and after being deflected by the deflecting unit,
Light is guided onto the surface to be scanned via the image forming means, and the surface to be scanned is optically scanned.

In particular, (2-1) the light source means comprises a vertical oscillation type surface emitting laser, a light emitting diode (LED), or a super luminescent diode (SLD); The optical system has a convex lens, (2-3) a beam expander or a relay lens is arranged in the imaging optical system, and (2-4) the convex lens is a light emitting surface of the light source means. In contrast, the Bessel beam is generated at a position separated by the focal length of the convex lens and near the position separated by the focal length of the convex lens.

[0017]

FIG. 1 is a schematic view showing a main part of an optical system according to a first embodiment of the present invention for generating a Bessel beam, and FIG. 2 is an enlarged explanatory view of the light source means shown in FIG.

1 and 2, reference numeral 12 denotes a light source means for emitting a light beam (laser beam) having a cylindrical light intensity distribution, for example, a vertical oscillation type surface emitting laser (semiconductor laser).
The vertical oscillation type surface emitting laser 12 is disclosed by Ito et al. In a magazine O plsu E, January 1987, p69-72. Reference numeral 11 denotes a laser beam (light output) having a cylindrical (donut-like) light intensity distribution. Reference numeral 22 denotes a convex lens serving as an imaging optical system. The convex lens 22 is disposed at a position apart from the laser end surface (light emitting surface) of the vertical oscillation type surface emitting laser 12 by a focal length (-f) of the convex lens 22. The Bessel beam is generated in an interference area (Bessel beam area) near a position separated by the focal length (+ f) of the convex lens 22. Reference numeral 21 denotes an interference region (Bessel beam region), which is a zeroth-order Bessel function of the first kind, 2
A Bessel beam having an intensity distribution approximately proportional to the power is formed.

Conventionally, a laser beam (near-field image) emitted from a light emitting point of a gas laser or a semiconductor laser as a light source used for Bessel beam formation usually has a Gaussian distribution. On the other hand, the laser beam (near-field image) emitted from the emission point of the vertical oscillation type surface emitting laser 12 used in the present embodiment has a cylindrical (donut-shaped) light intensity distribution and is far from the emission point. The laser beam (far-field image) at a distant point also has a cylindrical (donut-like) light intensity distribution and tends to diverge. This is because the beam spreads in the width direction of the donut shape because the active layer forming the light emitting point of the vertical oscillation type surface emitting laser 12 has a narrow circular shape.

In the present embodiment, a donut-shaped laser beam (light output) 11 emitted from a vertical oscillation type surface emitting laser 12 passes through a convex lens 22 and
A Bessel beam is formed in an interference area (Bessel beam area) 21 near a position separated by a focal distance (+ f) of 2. The light intensity distribution of the Bessel beam formed in this manner is determined by the vertical oscillation type surface emitting laser 12 and the convex lens 2.
2 has a rotationally symmetric intensity distribution because it is rotationally symmetric with respect to the optical axis.

In this embodiment, as described above, the convex lens 2
2 with respect to the laser end face of the vertical oscillation type surface emitting laser 12.
By arranging the convex lens 22 at a position away from the focal length (-f) of the convex lens 22, the conventional convex lens can
The center intensity of the spot can be further concentrated on the optical axis as compared with the case where the convex lenses are arranged apart by the focal length (-2f).

In this embodiment, a relay lens may be arranged in the optical path behind the interference area (Bessel beam area) 21, so that the depth of focus of the Bessel beam can be further increased.

As described above, in this embodiment, the laser beam emitted from the light source means (vertical oscillation type surface emitting laser) for emitting the laser beam having the cylindrical light intensity distribution as described above is incident on the imaging optical system (convex lens). By doing so, it is possible to generate a Bessel beam having an intensity distribution substantially proportional to the square of the 0th-order Bessel function of the first kind,
In particular, in the present embodiment, the vignetting of the light beam is small, the light amount of the light source can be effectively utilized, and the Bessel beam can be easily generated with a simple optical system.

In this embodiment, a vertical oscillation type surface emitting laser is used as the light source means. However, the present invention is not limited to the vertical oscillation type surface emitting laser, but an optical element which emits a light beam having a cylindrical light intensity distribution, for example, an LED. The present invention can be applied in the same manner as in the first embodiment even if a (light emitting diode), an SLD (super luminescent diode), or the like is used.

FIG. 3 is a schematic view showing a main part of an optical system according to a second embodiment of the present invention for generating a Bessel beam. In the figure, the same elements as those shown in FIG. 1 are denoted by the same reference numerals.

[0026] The foregoing embodiment 1 differs in this embodiment in the optical path between the vertical oscillation type surface emitting laser and a convex lens, for example, focal length second first lens and the focal length f ₂ of f ₁ Beam expander 31 composed of lenses
Are arranged to form an imaging optical system. Other configurations and optical functions are substantially the same as those of the first embodiment, and thus the same effects are obtained.

[0027] That is, 10 is an imaging optical system in the figure, the first lens 31a and the focal point of the focal length f ₁ length f ₂
And a convex lens 22 having the second lens 31b.

In this embodiment, the beam diameter of the donut-shaped laser beam (light output) 11 emitted from the vertical oscillation type surface emitting laser 12 is expanded by the beam expander 31 to reduce the diffraction loss and pass through the convex lens 22. , An interference region (vessel beam region) 32 forms a Bessel beam. The light intensity distribution of the Bessel beam thus formed is determined by the vertical oscillation type surface emitting laser 1.
2. Since the beam expander 31 and the convex lens 22 are rotationally symmetric with respect to the optical axis, they have a rotationally symmetric intensity distribution.

As described above, in the present embodiment, by arranging the beam expander 31 in the optical path between the vertical oscillation type surface emitting laser 12 and the convex lens 22 as described above, the focal depth of the Bessel beam in the interference area can be effectively reduced. Can be deeper.

FIG. 4 is a sectional view (main scanning sectional view) of a main part in the main scanning direction of the first embodiment when the method of generating a Bessel beam according to the present invention is applied to an optical scanning device.

In FIG. 1, reference numeral 12 denotes a light source means for emitting a laser beam having a cylindrical light intensity distribution, which comprises, for example, a vertical oscillation type surface emitting laser. Reference numeral 11 denotes a laser beam (light output) having a cylindrical (donut-like) light intensity distribution.

Reference numeral 22 denotes a convex lens serving as an imaging optical system. The convex lens 22 is located at a distance from the laser end surface (light emitting surface) of the vertical oscillation type surface emitting laser 12 by a focal length (-f) of the convex lens 22. And a Bessel beam is generated in an interference area (vessel beam area) near a position separated by a focal length (+ f) of the convex lens 22.

Reference numeral 5 denotes a condenser lens as a condenser means.
It is composed of a single lens, and focuses the Bessel beam and makes it incident on the deflecting surface 6a of the optical deflector 6 as deflecting means. The optical deflector 6 is composed of, for example, a rotating polygon mirror (polygon mirror) having four deflecting surfaces, and is rotated at a constant speed in a direction indicated by an arrow C around a rotation axis O by driving means (not shown) such as a motor. doing.

Reference numeral 7 denotes an fθ lens as an image forming means.
The laser beam is composed of two spherical lenses 7a and 7b. The laser beam deflected and reflected by the optical deflector 6 is
An image is formed in the vicinity. The scanned surface 8 corresponds to a photosensitive drum surface in, for example, a copying machine or an LBP (laser beam printer).

In this embodiment, a donut-shaped laser beam (light output) 11 emitted from a vertical oscillation type surface emitting laser 12 enters a convex lens 22. The laser beams emitted from the convex lens 22 interfere with each other,
A Bessel beam B1 having an intensity distribution substantially proportional to the square of the zeroth-order Bessel function of the first type is formed near a plane (position) A crossing the optical path. And Vessel beam B
1 diverges away from the plane A,
As a result, the light is condensed near the deflection surface 6a of the optical deflector 6. The laser beam deflected and reflected by the optical deflector 6 is converged by the fθ lens 7 to form a Bessel beam B2 on the surface 8 to be scanned. still,
In the present embodiment, the plane A and the scanned surface 8 are set to have an optically conjugate relationship.

Next, by rotating the optical deflector 6 in the direction of arrow C in the figure, the surface to be scanned 8 is optically scanned in the main scanning direction with a small spot diameter as shown by arrow D in the figure, and the surface 8 to be scanned is moved. The image is optically scanned (recorded) two-dimensionally by moving or rotating in the sub-scanning direction.

As described above, in this embodiment, the laser beam formed near the surface 8 to be scanned is a non-diffracting beam, and the non-diffracting beam has a very deep focal depth as described above. Even if the curvature of field of the optical system is large, or if there is an error or variation in the arrangement position of the optical components, the image does not deteriorate, and a high-quality image can always be obtained.

In this embodiment, the vertical oscillation type surface emitting laser is used as the light source means. However, the present invention is not limited to the vertical oscillation type surface emitting laser, but an optical element which emits a light beam having a cylindrical light intensity distribution, for example, an LED. The present invention can be applied in the same manner as in the above-described embodiment even if (light emitting diode), SLD (super luminescent diode) or the like is used.

In this embodiment, a relay lens may be arranged in the optical path behind the interference area (Bessel beam area), and the beam expander may be arranged in the optical path between the vertical oscillation type surface emitting laser 12 and the convex lens 22. May be provided to constitute the optical system.

In this embodiment, the method of generating a Bessel beam is applied to an optical scanning device. However, the present invention is not limited to this, and can be applied to a wide range of optical devices such as an optical storage device.

[0041]

According to the present invention, a Bessel beam having a large depth of focus is generated by making the light beam emitted from the light source means for emitting a light beam having a cylindrical light intensity distribution incident on the imaging optical system as described above. In particular, a Bessel beam generation method and an optical scanning device using the same that can generate a Bessel beam easily with a simple optical system while effectively utilizing the light amount of a light source with less vignetting of a light beam. can do.

[Brief description of the drawings]

FIG. 1 is a schematic diagram illustrating a main part of an optical system according to a first embodiment of the present invention that generates a Bessel beam.

FIG. 2 is an enlarged explanatory view of the light source means shown in FIG.

FIG. 3 is a schematic diagram illustrating a main part of an optical system according to a second embodiment of the present invention that generates a Bessel beam.

FIG. 4 is a sectional view of a main part in the main scanning direction of the first embodiment when the method of generating a Bessel beam according to the present invention is applied to an optical scanning device.

FIG. 5 is a schematic view of a main part showing an optical system for generating a conventional Bessel beam.

FIG. 6 is a main part schematic diagram showing an optical system for generating a conventional Bessel beam.

FIG. 7 is an enlarged explanatory view of the light source means shown in FIG. 6;

FIG. 8 is a main part schematic diagram showing an optical system for generating a conventional Bessel beam.

[Explanation of symbols]

 Reference Signs List 11 light output (laser beam) 12 light source means (vertical oscillation type surface emitting laser) 21, 32 interference area (Bessel beam area) 22 convex lens 10, 30 imaging optical system 31 beam expander 31a first lens 31b second Lens 5 Condensing means (condensing lens) 6 Deflection means (rotating polygon mirror) 7 Imaging means (fθ lens) 8 Scanning surface

Claims (10)

[Claims] 1. A light beam emitted from a light source means for emitting a light beam having a cylindrical light intensity distribution is incident on an imaging optical system, so that the intensity is substantially proportional to the square of the first-order zero-order Bessel function. A method for generating a Bessel beam having a distribution. 2. A method according to claim 1, wherein said light source means comprises a vertical oscillation type surface emitting laser, a light emitting diode (LED), or a super luminescent diode (SLD). 3. The method according to claim 1, wherein said imaging optical system has a convex lens. 4. The method according to claim 3, wherein a beam expander or a relay lens is provided in the imaging optical system. 5. The convex lens is disposed at a position away from a light emitting surface of the light source means by a focal length of the convex lens.
4. The method according to claim 3, wherein said Bessel beam is generated near a position separated by a focal length of said convex lens. 6. A light beam emitted from a light source means for emitting a light beam having a cylindrical light intensity distribution has an intensity distribution substantially proportional to the square of a first-order zero-order Bessel function via an imaging optical system. A Bessel beam is collected, the Bessel beam is condensed by a condensing means, made incident on a deflecting means, deflected by the deflecting means, and then guided on a surface to be scanned through an image forming means. An optical scanning device characterized in that an optical scanning is performed. 7. The optical scanning device according to claim 5, wherein said light source means comprises a vertical oscillation type surface emitting laser, a light emitting diode (LED), or a super luminescent diode (SLD). 8. The method according to claim 6, wherein the imaging optical system has a convex lens. 9. The method according to claim 8, wherein a beam expander or a relay lens is provided in the imaging optical system. 10. The method according to claim 1, wherein the convex lens is disposed at a position away from a light emitting surface of the light source means by a focal length of the convex lens, and generates the Bessel beam near a position away from the focal length of the convex lens. 9. The method according to claim 8, wherein: